It depends on what you mean. If you are referring to the moral gravity of an individual instance committed by a married couple, then abusing NFP is usually less serious than using artificial contraception.
However, if you mean "Which is worse in terms of destroying an authentically Catholic concept of marriage?" then I would answer "NFP." The reason is that everyone recognizes that the pill and condoms and so forth are not Catholic. Even the 97% of married Catholic couples who have used such devices are aware that they are contrary to Catholic teaching. They know that they are sinning.
But NFP, on the other hand, is presented as a positive good, when it is not. It is at best a concession. But look at the propaganda being spewed by the USCCB: "A Way of Life, a Way of Love." Clearly they do not see NFP as reserved for grave reasons. So they are doing much more damage to the authentically Catholic understanding of generosity and fruitfulness and "accepting children in whatever number God chooses to send them."
A subtle heresy that contains some orthodoxy is always much more dangerous than a blatant heresy that clearly contradicts the truth of the faith.
Unfortunately, being aware that artificial forms of birth control are contrary to Church teaching doesn't necessarily mean that people know they're sinning if they use them. At least not in today's more secularized world, very sorry to say.
So they are doing much more damage to the authentically Catholic understanding of generosity and fruitfulness and "accepting children in whatever number God chooses to send them."
How is this so? What do they say to put out this message?